There are known apparatuses which make possible to decrease the contents of foreign materials of fluids by an arrangement in which the fluid streams along a thin membrane filter while the pressure of the fluid is higher then the pressure existing on the other side of the membrane filter. The streaming fluid is generally a solution and one part of it is pressed through the membrane filter (permeate). The composition of the permeate differs from the composition of the original solution because the permeability of the membrane filter is different for ions and particles of different sizes.
If the method utilizes only the fact that the size of pores of the membrane filter lies in the order of sizes of ions or particles and so the membrane filter filters (withdraws) the the bigger ions and particles in some way, then the method is called ultra or microfiltering. If there is a significant difference of osmotic pressure between the two sides of the membrane filter, then after having equalized that and bringing up an opposite pressure difference, higher than it has been there previously, a phenomenon reverse to the usual osmosis, the so called reverse osmosis appears. The solvent streams through the membrane filter from the solution against the osmosis pressure to the space containing solvent. In the operation of the membrane filter equipment the energy consumption is determined mostly by the pump necessary to feed the fluid to be treated (i.e. raw fluid) to the equipment under high pressure (e.g. 50-70 bars). In order to decrease the energy consumption there have been proposed an arrangement in which a water turbine is driven by the high pressure concentrated fluid (concentrate) leaving the membrane filter equipment for getting back a part of the energy. However, in the practice this solution needs great investment and the quick rotating water turbine makes the operation and maintenance of the whole water purification system complicate.
Energy recovery is objected also by the solution described in U.S. Pat. No. 4,354,939. According to this the concentrate leaving the membrane filter equipment is fed to two containers alternatively. Each container has two spaces separated from each other by a flexible membrane. When the concentrate gets into the first space of one of the containers, a pump feeds back raw fluid from the second space to the inlet of the membrane filter equipment. When the first space, namely almost the whole container has been filled with concentrate, the concentrate is being fed to the other container, and meanwhile from said one of the containers the concentrate is being discharged by the raw fluid fed to the second space of this container.
In said membrane filter equipments the raw fluid streams along the membrane filter. From the point of view of the permeate-flow through the membrane the concentration of two boundary layers situated closely to both sides of the membrane is determinative. The concentration of the boundary layer is always higher than the mean concentration of the whole solution-flow. Namely, the solvent passes from the inside of the solution towards the membrane filter which withdraws the mass of the solved materia but lets the solvent get through, therefore the solved materia becomes concentrated in the boundary layer. The difference of the osmotic pressure is proportional to the difference of the concentrates in the boundary layers. Because of the phenomenon described the osmotic pressure increases and thus the energy consumption increases, too. In the case of ultra or microfiltration the hydralic resistance of the membrane filter increases because of the increase in the concentration of the solution fed into the membrane filter (this appears obviously in the case of revers osmosis, too and causes a further increase in the pressure). The phenomena mentioned above are frequently called polarization, too. The thickness of the boundary layer and so the concentration increase diminish if the speed of the solution (more exactly the R number proportional to the speed) becomes higher. Solvent is leaving the solution which is flowing along the membrane filter. At the end of the membrane filter, at the outlet, the mean concentration is equal to the concentration of the solution just leaving the equipment. If the quantity of the solution fed into the equipment is n times the quantity of the permeate pressed through the membrane, consequently the so called gain is 1/n, then at the outlet the mean concentration will be n/(n-1) times higher than at the inlet. Because of the mentioned relation between the concentration and the osmotic pressure it would be advantageous if n was higher, but by this the gain (1/n) would decrease.
It has been proposed that for decreasing the thickness of the boundary layer the raw fluid should be circulated through the membrane filter equipment. The raw fluid is continuously fed by a high pressure pump into the circle of fluid circulated and from this on one hand the permeate and on the other hand the fluid concentrated leave continuously. A disadvantage of this method is that the concentrate of the fluid fed into the membrane filter equipment will be higher than that of the raw fluid and thus the increase in the osmotic pressure results in just an opposit effect. Consequently, it is worthful to let the fluid circulated in this way only if the advantage caused by the fact that the boundary layer becomes thinner is greater than the disadvantage caused by the increase in the concentration. This method has reason when the concentration of the raw fluid is very small. But this method is not advantageous for the treatment of solutions of higher concentration e.g. for desalination of sea-water. The energy of the concentrate continuously leaving the membrane filter equipment gets lost unless there is any other way for recovering it, e.g. by a water turbine.
In the methods mentioned above the membrane filter equipment is of a steady state operation thus the concentration and the speed of the fluid flowing in it depends only on the place. The speed of flow has to be chosen suitable to the concentration of the raw fluid and to the maximal concentration allowed for the membrane filter equipment.